1. Field of the Invention
The present invention relates to a Raman amplifier and an optical transmission system using the amplifier, and in particular, relates to a technology for extending the wavelength bandwidth used to transmit signal light.
2. Description of the Related Art
Conventionally, in a long-haul optical transmission system, at each relay device, signal light was once converted into an electrical signal for 3R-process (Re-timing, Re-shaping and Re-generating), then the electrical signal was converted back into signal light and the signal light is transmitted to a next relay device. However, an optical amplifier for amplifying optical signal without converting the signal into an electrical signal has been promoted and a transmission system using the optical amplifier as a linear relay device has been studied. By replacing a repeater that requires the optical/electrical conversion described above with an optical amplifier/repeater, it is expected that the number of parts composing each relay device is greatly reduced, reliability is improved and cost is reduced accordingly.
As the Internet and the like have been spread, the amount of information to be transmitted through a network has increased and a technology for improving the capacity of a transmission system has been actively studied. As one method for implementing the large capacity of a transmission system, WDM (wavelength division multiplex) optical transmission method is focused. In a WDM transmission system a plurality of signals are multiplexed and transmitted using a plurality of carrier waves with different wavelength, and an information amount to be transmitted through one piece of optical fiber can be greatly increased.
FIG. 1 shows the configuration of a general optical transmission system. In this system, multi-wavelength light is transmitted from an optical transmitter 100 to an optical receiver 200. Specifically, the optical transmitter 100 generates multi-wavelength light by multiplexing a plurality of signal lights with different wavelengths and transmits the multiplexed signals to a transmission line. The optical receiver 200 detects each signal by demultiplexing the received multi-wavelength light for each wavelength. The transmission line is made from an optical fiber and optical amplifiers are installed at predetermined intervals.
For each optical amplifier, usually an erbium-doped fiber amplifier (EDFA) is used. The gain wavelength band of a general EDFA is 1.55 xcexcm and that of a GS-EDFA (Gain Shift EDFA) is 1.58 xcexcm. Each of the bandwidth of the EDFAs is approximately 30 nm. Therefore, if an EDFA is provided in the transmission line of a WDM optical transmission system, a plurality sets of signal light are transmitted using carrier waves within this gain wavelength band.
To increase the capacity of a transmission system, it is effective to increase the number of wavelengths to be multiplexed. Lately, as an optical amplification method for a wider gain wavelength band compared with an EDFA, a Raman amplifier using Raman scattering is focused.
In Raman amplification, when a pump light is provided to an optical fiber, a gain is obtained at a wavelength which is longer than the wavelength of the pump light. For example, in a case of GeO2-doped silica (SiO2) optical fiber, in a 1.55 xcexcm band, again is obtained at a wavelength which is longer by approximately 100 nm compared with the wavelength of pump light, as shown in FIG. 2A. This amount of shift is 13.2 Tera Hz, when converted into a frequency. A Raman amplifier can amplify an arbitrary wavelength if only a corresponding pump light can be prepared.
A Raman amplifier is implemented by using the above described nature. To obtain a wide gain wavelength band, a plurality of pump lights with different center frequencies are used, as shown in FIG. 2B. This method is recited, for example, in Y. Emori et al., xe2x80x9c100 nm Bandwidth Flat Gain Raman Amplifiers Pumped and Gain-equalized by 12-wavelength Channel WDM High Power Laser Diodesxe2x80x9d, OFC""99 PD19, 1999. In this way, by using a plurality of pump lights, a wider gain wavelength band can be obtained.
FIG. 3 shows a configuration of a WDM optical transmission system using a Raman amplifier. A pump light for Raman amplification is basically supplied to a transmission line optical fiber so as to be transmitted in the opposite direction of signal light. In this case, if a plurality of pump lights are used, as shown in FIG. 2B, pump lights output from a plurality of light sources with different oscillation frequencies are supplied to a transmission line optical fiber by a wavelength coupler (wavelength multiplexer) or the like.
As described above, a Raman amplifier using Raman scattering obtains a gain in a wavelength band which is longer by approximately 100 nm than a wavelength of pump light in a 1.55 xcexcm band. For example, as shown in FIG. 4, when a pump light P1 with wavelength xcex1 is inputted, a gain is obtained in a wavelength band which is longer by approximately 100 nm than wavelength xcex1 (around wavelength xcex3). Similarly, when a pump light Pk with wavelength xcex2 is inputted, a gain is obtained in a wavelength band which is longer by approximately 100 nm than wavelength xcex2 (around wavelength xcex4) Therefore, if a plurality of pump lights P1 through Pk are appropriately used, a gain bandwidth of appropriately 100 nm can be obtained. In this case, a plurality of signal lights S1 through SL are transmitted using this gain wavelength bandwidth of appropriately 100 nm.
However, in a conventional Raman amplifier, gain bandwidth is limited by the respective shift amounts from a wavelength of pump light to a wavelength of Raman gain obtained due to the pump light. Specifically, if the difference in wavelength between pump light and Raman gain obtained due to the pump light is 100 nm, the maximum gain bandwidth obtained by a Raman amplifier is also approximately 100 nm.
It is an object of the present invention to provide a Raman amplifier which obtains a wider gain bandwidth.
A Raman amplifier of the present invention amplifies multi-wavelength light including a plurality of signal lights, and comprises: a transmission medium for transmitting the multi-wavelength light and first pump light for amplifying the multi-wavelength light; a light source for generating second pump light for amplifying the multi-wavelength light; and an optical device for guiding the second pump light generated by the light source to the transmission medium, wherein at least one of the first pump light and second pump light is located within the band of the multi-wavelength light.
According to this Raman amplifier, if the first pump light is appropriately located within the band of the multi-wavelength light and the second pump light is appropriately located at a wavelength which is shorter than that the multi-wavelength light, the second pump light amplifies a part of the signal light and first pump light, and the amplified first pump light amplifies the other part of the signal light. Thus, Raman amplification with a wider band than a Raman shift amount can be realized.
A Raman amplifier in another feature of the present invention amplifies multi-wavelength light including a plurality of signal lights, and comprises: a transmission medium for transmitting the multi-wavelength light and auxiliary light with a longer wavelength than the wavelength of the multi-wavelength light, a light source for generating pump light for amplifying the multi-wavelength light, and an optical device for guiding the pump light generated by the light source to the transmission medium.
According to this Raman amplifier, the multi-wavelength light is amplified by the pump light. The auxiliary light absorbs a part of the energy of the amplified multi-wavelength light. Therefore, the multi-wavelength light can be avoided from being too strong.
An optical transmission system of the present invention has a configuration in which multi-wavelength light including a plurality of signal lights is amplified by a Raman amplifier, wherein the Raman amplifier amplifies the multi-wavelength light using a plurality of pump lights located at frequency intervals of 1/n of a Raman shift amount (n is an integer).
According to this transmission system, since a plurality of pump lights for Raman amplification are located at frequency intervals of 1/n of a Raman shift amount, the peak frequency of Raman gain due to particular pump light matches the frequency of corresponding pump light. Therefore, it is easy to adjust a power of each of the pump lights for equalizing Raman gain.